A common use of equipment racks of the type to which the present invention relates is for carrying telephone circuitry in a building. In general, such racks include a base, side members extending upwardly from the base, and an upper horizontal cross member connecting top ends of the side members. Equipment is mounted between the side members and wire and cable connected to the equipment is supported by the rack. Typically, each rack carries up to 400-500 pounds of equipment, and is bolted or otherwise secured to the floor.
Preservation of electronic equipment during earthquakes and aftershocks for the maintenance of communications and other purposes is a major concern of earthquake preparedness. In fact, alleviation of damage and suffering could depend on the maintenance of the telephone system and broadcast facilities. Moreover, the introduction of electronic and fiber optic telephone switching equipment has significantly increased the density of calls being handled in a single equipment rack or network bay such that thousands of telephone lines can be interrupted with the loss of one bay of equipment. This has made the reliability of telephone switch equipment and its supporting structure critically important, especially with respect to earthquake resistance. Racks of this type, therefore, are required to meet minimum standards for earthquake resistance, such as the BELLCORE Zone 4 seismic test set forth in Document #GR-63-CORE.
It is possible to make an equipment rack that is strong enough to avoid earthquake damage by using heavier material and more bracing members, but this adds significantly to the cost. Heavier material and more bracing, therefore, are not a solution acceptable in the competitive environment of modem telephone systems. In addition, it has been found that lighter weight, yet more rigid equipment racks behave more favorably during shaking of the type encountered during earthquakes.
During seismic motion the base of a tall, slender rack moves with the floor to which it is anchored. If the rack is sufficiently rigid and well anchored it will closely follow the motions of the base and floor. If, however, the rack is more flexible, it will move at a rate different to that of the base and floor, and consequently experience high stresses and deflections. Free-standing electronic equipment racks typically have low natural resonance frequencies in the range from 1 to 10 Hz. In earthquakes, the highest energy dissipation occurs in the 4 to 5 Hz region, thereby making the racks vulnerable to earthquake induced damage. Increasing the natural frequency of the racks above the 4 to 5 Hz range, therefore, has been found to improve the earthquake resistance of equipment racks. One way to increase the natural frequency of an equipment rack is to increase its rigidity without increasing its weight.
What is still desired is a new and improved telecommunications equipment rack that is earthquake resistant. Preferable, the improved rack will include increased structural rigidity without a substantial increase in weight.